Method of effecting an austenite to martensite transformation in a sustained intensity magnetic field



4 Sheets-Sheet l June 8, 1965 w. L BAssETT nl METHOD OF EFFECTING AN AUSTENITE TO MARTENSITE TRANSFORMATION IN A SUSTAINED INTENSITY MAGNETIC FIELD Filed Oct. 28, 1960 June 8, 1965 w. l. BAssETT In METHOD OF EFFECTING AN AUS'IENITE TO MARTENSITE TRANSFORMATION IN A SUSTAINED INTENSITY MAGNETIC FIELD 4 Sheets-Sheet 2 Filed Oct. 28, 1960 INVENTOR. ML L/AM [A9/155577,27 BY Wag@ 47m/wey June 8, l1965 v w. l. BAssETT nl 3,188,248

METHOD OF EFFECTING AN AUSTENTE TO MARTENSITE TRANSFORMATION IN A SUSTAINED INTENSITY MAGNETIC FIELD Filed OO-t. 28, 1960 4 Sheets-Sheet 5 .PV/1 L/AM I BASSE TT, E

Afro/wey United States Patent O 3,138,243 WTHQD F EFFECTING AN AUSTENITE T0 MARTENSITE TRANSFORMATION IN A SUS- TAENED INTENSTY MAGNETIC FIELD William I. Bassett IH, Los Angeles, Calif. (120 W. 154th St., Gardena, Calif.) Filed Oct. 28, 1960, Ser. No. 65,358 7 Claims. (Cl. 148-108) This application is in part a continuation of my application S.N. 842,427, filed September 25, 1959, now abandoned, for Method and Apparatus for Heat Treating Metals.

This invention relates to the heat treatment of alloys having an austenitic to martensitic transformation, and has as its basic object to improve the tensile and uniformity characteristics of such alloys in relation to the plasticity thereof. The invention is especially applicable to the conditioning of magnetic alloys having the austeniticmartinsitic type phase change. The term plasticity is used in the broad sense to include all of the desirable properties of ductility and malleability, such as the ability to withstand bending and stretching, as in forming operations; rnachinability, i.e., the adaptability to cutting and abrading in shaping and finishing operations; and in particular the ability to undergo the removal of metal smoothly with a minimum of tearing action; and formability, i.e. the ability to undergo a high degree of deformation, as in eoining operations, various impact extrusion operations and miscellaneous forming operations wherein considerable proportions of the alloy are moved from one area to another, without being fractured or weakened in the process.

Another object is to provide an improved heat-treating process and apparatus whereby improved tensile strength may be imparted to alloys without incurring the loss of ductility, malleability, etc. which normally is regarded as an inevitable consequence of the process o f increasing tensile strength. Inversely, the invention aims to provide a heat treated alloy article having improved plasticity, i.e. ductility, formability, machinability, together with improved tensile strength, elasticity, and allied properties. Stated more specifically, the invention provides for attaining the ductility and malleability normally attained only by a considerable sacrifice in tensile strength, while simultaneously attaining the high tensile strength and elasticity which can be attained, in conventional hardening processes, only through an extreme sacrifice in ductility, machinability etc., and which, under normal processing methods, are partially restored to the alloy by a tempering process wherein the hardness and tensile proprties are reduced in order to improve the ductility. Y

A further object is to provide an improved heat treating method and apparatus, wherein in addition to the greatly improved tensile strength in relation to plasticity referred to above, there is attained a uniformity and reliability of result going far beyond the uniformity and reliabiilty that can be attained in known processes. This is an extremely important characteristic of the invention, since it will be immediately apparent thatrwhere a process has iiuctuating results, it is no better than the minimum point in its range of fluctuation, since it cannot be relied upon for results above that minimum point. To be more specific, the invention provides a heat-treating process the results of which are of greatly increased uniformity over the results of conventional processes.

A further object is to provide an improved heat-treating process and apparatus resulting in a greatly increased reduction in stress concentrations in the treated metals,

which normally greatly reduces the yield characteristic of v the metal, i.e. the likelihood of fracture or other weaken- 3,188,248 Patented June 8, 1965 ing occurring therein when subjected to stress and strains in use. More specifically, the invention attains a very significant reduction in residual stresses arising through hardening processes.

A further and related object is to provide an improved process and apparatus by which stress relieving is accomplished at much lower temperatures than are utilized in normal tempering or stress-relieving operations. l

In general, the invention provides a process wherein, following the initial heating stage, quenching takes place in the presence of a sustained magnetic eld which funetions to maintain high plasticity characteristics in the alloy while imparting greatly increased tensile properties thereto. The process, in some of its applications, tends to inhibit the growth of long-needleV type crystals, which ordinarily takes place during and after the quenching step.

The invention is particularly characterized by the application of a magnetic field to an alloy in a quenching step during the transition through the critical range when the austenitic-to-martensitic phase transformation is taking place, and the attainment of a substantially instantaneous reaction of the magnetic field upon the alloy crystals in the critical range in the phase transformation. It is important that the magnetic field be continuously applied without reversal of polarity (eg. as the result of a direct current magnetizing force). Y

For optimum results, the primary (magnetic) quench is applied at an elevated temperature which, for most alloys, is in a range from 250 F. to 1100 F., and is followed by a quick secondary quench to approximately room temperature, preferably in water or brine, prior to the refrigerative fixing quench.

The invention further contemplates a succeeding processing step, following the magnetic quenching step, wherein the combination of high-tensile strength, high plasticity characteristics imparted to the alloy in that step, are'xed by lowering the temperature of the alloy to an extremely low temperature range as low as or lower than l--" F.

As a further important step in the process, following the low temperature treatment, the alloy is subjected to a low temperature stress-relieving or age-tempering step in which itis heated to a temperature lower than a conventional range of stress-relieving temperatures for the the particular alloy being processed, and is maintained at that temperature for a duration such as to restore maximum ductility. The alloy is then air-cooled.

Y It has been found that for some alloy compositions magnetic quenching is effective at temperatures above room temperature, while forv other alloy compositions, magnetic quenching at temperatures below r-oom temperature, is effective. Y

The basic process, in its optimum embodiment, includes the following steps:

(1) Austenitizing (heating the alloy to a temperature well above the upper critical temperature).

(2) Primary (magnetic) quench through the transformation range. v

(3) Secondary (quick) quench to approximately room temperature. l

(4) Tertiary (refrigerative) quench.

(5) Stress relief. Y

I find that the grain structure as fixed in my process, does not undergo any noticeable further transformation even over many months of observation, and accordingly, I have concluded that the combination of processing steps in my process results in a stable, stress-relieved vcondition in the metal which remains permanent.

Other objects and advantages will become apparent in the ensuing specifications and appended drawings in which: Y Y

FIG. l is a Vlongitudinal vertical sectional View of an Y apparatus embodying one form oftheV invention;

FIG. 2 is a vertical longitudinal sectional view of an apparatus embodying a modified form o-f the invention;

FIG. 3 is a horizontal sectional view of the magnetic quench tank thereof, taken on the line 3 3 of FIG. 2;

FIG. 4 is a horizontal sectional View of a modified form of the quench tank;

FIG. 5 is a vertical longitudinal sectional view of a heat treating apparatus embodying another modified form of the invention;

PIG. 6 is a plan view of the same, partially in horizontal section as indicated by the line 6 6 of FIG. 5; and

FIG. 7 is a chart of the process, illustrating in particular the change in temperature level 'between the several stages of the process.

Referring now to the drawings in detail, I have shown therein, as a few selected examples of apparatus that may 'be `employed in the practice of my process, a general arrangement of heat treatment units which in each case include a furnace for initially heating the metal; a magnetic quench unit in which the metal is subjected to the action of a magnetic field while being quenched to an intermediate temperature; a secondary quench unit wherein the temperature of the metal is reduced to a temperature considerably below the said intermediate temperature of the magnetic quench step; and a refrigerating unit wherein the final low-temperature quench fixing the molecular structure is effected. i

In FIG. 1, the heating furnace is indicated generally at A, the magnetic quench unit is indicated generally at B, the secondary lquench unit (which may be any conventional liquid quench), is indicated in block diagram at C, and the refrigerator unit in which the final quenching step takes place (which likewise can utilize any conventional refrigerator apparatus adapted to produce an atmosphere down to 150 F.) is indicated in block diagram at D.

THE APPARATUS IN ITS PREFERRED FORM-FIG. l

Referring now to FIG. 1 in detail, the apparatus may utilize a conventional roller type conveyor I5, having rollers 16 on which a basket or other container of parts to be heat treated, maybe pushed by means of a suitable push rod, Aor can be pulled by the use of a conveyor chain or the like, toward the furnace A as indicated by arrow 17.

The furnace A.-The furnace A, which can be lof any conventional type or construction, is shown by way of Villustrative example `as comprising a gas-ired or electric furnace having a hearth 13 in the bottom of a housing 19, having an entry-way 20 normally closed by a door 21 slidable vertically in suitable tracks 29 in response to the operation of any suitable power operated actuator means, e.g. a pair of air cylinder actuators indicated in broken lines at 22; having any suitable conventional control such as a thermo-couple 23 in, a heat insulating ceiling 24, (which may be of alumina brick); having a series of heating elements 25 (e.g. glow-bars, non-metallic) in the ceiling 24 and hearth 13, having a hearth plate 26 (eg. ceramic) providing asmooth, hard, high temperatureresistant upper surface for the hearth 1S, `over which the loaded containers can be easily slid; and having an exit 27, normally closed by a door 28 slidable vertically in ways 30 in the respective sides of the discharge end of the furnace. Powered actuator means 22', which may 'be similar to the actuator 22, is utilized for raising and lowering the door ZS,

Ahead of the door Z1 is a conventional iiame curtain unit 31 for heating the doorway 29 when the door 21 iS raised and thereby preserving the loss of temperature level within the furnace, and which also functions'to burn off the hydrogen issuing throughithe door during the period of opening thereof. At this point it should be noted that the invention utilizes an inert atmosphere Within the furnace A to completely inhibit oxidation or other corrosion processes during the 'heating step. Such, an atmosphere may be attained by pumping any inert gas (such as t@ hydrogen) into the furnace during the operation of the process, and such apparatus for doing this is well known and is not illustrated in the drawing.

Magnetic quench :mit B The magnetic quench unit B comprises a non-corrosive (eg. stainless steel) quench tank 32 having a bottom 33 and a lateral lining 34 which may be cylindrical (as indicated in FIG. 3 for the form of the invention shown in FIG. 2) or rectangular. Actually, the tank shown in FIG. 1 is intended to be understood as being rectangular, of greater width (transversely of the axis of movement of the work through the apparatus) than width (along said axis, i.e., the dimension shown in the drawing). The tank is surrounded by a relatively thick wall of insulation 35 (eg. vermiculite or any `suitable hydrous silicate) enclosed between an intermediate shell 36 surrounding the tank 35 and an outer housing 37 entirely surrounding the shell 36. Defined between the tank 34 and the intermediate shell 35, and entirely surrounding the tank horizontally, is a magnet chamber in which is disposed a series of electromagnetic coils 38, which, -for the rectangular tank 34, are of corresponding rectangular ring form. Although shown slightly spaced vertically, in order to schematically indicate electrical connections 39 extending to them in parallel, the several coils 3S can as well be stacked snugly one upon the other, and in fact are preferably arranged as compactly as possible so as to attain a magnetic field of maximum intensity permeating the tank 34 and the quenching chamber et? defined therein. A suitable source of direct current of high amperage (not shown) is provided to power the coils 38. To minimize loss of magnetic field, the tank 3ft is of a non-magnetic material and, for a satisfactory combination of non-magnetic characteristic and non-corrosive properties, the stainless steel alloy with high nickel content, known commercially as Inconel may be utilized.

A bath of quenching liquid (not shown) is maintained in tank 32 preferably at a level which is just sutiiciently below the top of the tank to prevent splashing over the rim 55 of the tank. Such quenching bath may be a typical fused body of salt. For maximum efficiency of heat exchange between the bath and the work parts being quenched, and between the bath and cooling coil 41, the liquid in tank 32 is agitated by suitable agitating means lsuch as a pair of impellers 57 (one at each lateral extremity of the quench chamber 40) each impeller being fixed to the lower end of and driven by a shaft 58 the upper end of which is driven by a suitable drive 59 (e.g. chain and sprocket) from a motor 6G located outside of the housing 49 and therefore shown in dotted lines. The two impellers S7 are operated in unison to circulate the bath vertically along the respective sides and bottom of the quench chamber.

At the discharge end of magnetic quench unit B is a roller conveyor 61 on which the baskets of treated parts can be pushed or pulled from the magnetic quench unit B to the secondary quench unit C. Similar conveyor apparatus may be utilized for facilitating the movement of the containers of work from the secondary quench unit C to the refrigeration unit D.

The broken lines 61 of FIG. 1 may beV understood as indicating a horizontal continuation of conveyor 61, and the broken line 62 is intended to indicate either a similar conveyor extending between the secondary quench unit C and the nal unit D or simply the path of movement of the work.

THE METHOD ri'ne operation of the apparatus of FIG. 1 will now be described as an incident to the following description of my Y improved method.

In many instances, the magnetic quench is followed by a secondary quick quench (eg. water quench) in the unit C, wherein a second drop in temperature level is effected. For many applications, the invention utilizes a succession of three quenching steps in three successively lower temperature ranges, in which a magnetic quench step is followed by a secondary quick quench step and then by a refrigerative quench in which the properties that have been imparted to the metal are fixed therein.

l am aware of course that others have hitherto suggested the possible advantageous use of a magnetic field in connection with the solidiiication of metals from a molten state and in connection with the passage of electric current through the metal, for the intended purpose of accentuating the 'growth of crystals and to increase the length thereof. insofar as I am aware, such prior proposals have either directed the use of alternating or liuctuating magnetic fields for the purpose of molecular agitation, and have failed to discuss the use of high intensity non-fluctuating (direct current-induced) magnetic fields at the temperature range of phase transformation for the purpose of improving the combination of tensile and plasticity properties.

(A) Azfstenilizz'ng.-In the initial step (hardening or austenitizing) performed in the furnace A, the metal parts are heated to a temperature between the upper critical temperature of austenitic transformation and a temperature several hundred degrees higher than the temperature range normally utilized in heat-treating processes. More specifically, the temperature of the work is raised to a temperature between about 1500 F. and 2500 F. in the heat-treating step, depending on the specific composition of the alloy.

(B) Initial quench-magnetic.-At the end of the austenitizing step, the work parts are transported through the doorway 2'7 at the temperature to which they were raised in heating chamber 14, but' will commence to lose temperature in the work chamber 50 which is maintained at a much lower temperature (eg. in most instances inthe neighborhood of 250 F.). To minimize this preliminary temperature loss, the work is moved onto the work table 42 as rapidly as possible and then is promptly lowered into the quench bath in the chamber 40, where the temperature is maintained at a slightly lower level than in the chamber 50. For example, for tool steels and refractory alloys, where the temperature in chamber 50 will be maintained approximately in the range of 400 F. to 800 F., the temperature in quench chamber 40 .'ill be maintained in a range substantially 50 lower. As a further example, for age-hardening stainless steels, the temperature in the quench 4bath will be maintained in a range of 250 F.-500 F. and in the upper chamber 50, in the range of 300 F. For nickel-chrome-molybdenum steels, quench chamber 40 will be maintained at a temperature in the range of 200 F. to 400 F. and for chrome-molybdenum steels, in the range of 200 F. to 500 F.

While other quenching mediums having comparable characteristics of heat-absorption rate and adaptability for maintaining an elevated quenching temperature within the range stated above, can be used, I prefer to use in the quench tank 40 a fused salt bath, for optimum results.

The work, having been deposited upon the table 42, the latter is promptly lowered into the tank 40, immersing the heated parts in the salt bath, and the temperature of the parts will then drop rapidly to approximately the temperature of the bath, the cooling coils 41 absorbing excess heat and carrying it away to maintain the bath temperature. This begins the. martensitic transformation stage. The work is retained Vin the salt bath 'for a period of time depending upon the alloy being treated, during which period substantial martensitic transformation progresses to the point where the transformation is essentially completed (although not yet fixed). This time interval for nickel-chrome type 4340 alloy steel (with relatively sol low percentage of alloying metals in addition to iron) may range betweeen ten minutes and twenty minutes depending upon the section size of the work being quenched (the ten minute minimum being applicable to any cross section below one inch diameter and the twenty minute maximum being applicable to Iany larger cross section within hardenability limits). For type 431 stainless steel, the duration may range between twenty minutes and one hour for a comparable limit of section sizes.

At a point, differing depending upon the composition of the alloy in the descending temperature of the alloy, the magnetic field, which is maintained as a sustained, non-fluctuating field throughout the period of quenching, becomes effective upon the developing martensitic crystals. Generally speaking, a fairly high intensitive magnetic field is necessary to the attainment of the results of the invention. Significant results have been obtained With magnetic fields having intensities as low as gauss,

Valthough it is preferable to use much higher intensity and optimum results have been obtained where field intensities in the higher range 750 gauss to 1500 gauss have been employed. It is believed that even higher intensities can be effectively employed.

(C) Secondary quench-The end-of the quenching period described above completes the stage of magneticquench processing for alloy compositions which undergo the desired phase transformation at relatively high temperatures. The work is then transferred through the door S1 upon the conveyor 61 or by any suitable conveying method of the secondary quench C. Preferably, this transfer is effected in a minimum period of time ranging below ten seconds and which may be as low as approximately one second.

In the secondary quench, the temperature of the parts is quickly reduced from the temperature of the magnetic quench bath down to room temperature. The secondary quench may utilize a conventional water bath, which, in view of the fact that the temperature of the parts is immediately dropped from the temperature of the parts as they leave the magnetic quench (eg. 300 F.) down to room temperature (eg. 70 F.) does not have any corrosive effect' upon the surface of the parts.

In the secondary quench, the transformation from austenitic to martensitic phase is substantially completed, although a few austenitic crystals may remain.

(D) Refrigerative quench.-The parts are then immediately transfered from the secondary quench C to the tertiary, refrigerative quench or transformation fix where- 1n any latent austenitic characteristic (eg. a minor percentage of austenitic crystals remaining after the completion of the martensitic transformation stage described above) are transformed and reduced to the martensitic state, and wherein the martensitic platelets become a permanently fixed, crystalline lattice. In the transformation fix D, the parts are subjected to a refrigerated atmosphere (e.g. air) at an extremely low temperature, in the range of -100 F. to *150 F., for various prolonged periods which may have a duration of three hours or more (a minimum of approximately three hours being desirable where no expediting medium is employed).

It is to be noted that some grades of steel have a transformation range extending as 10W as about room temperature (eg. between limits above and below room temperature). The invention contemplates magnetic quenching in such range. ln this case, themagnetic step may be preceded by a single quench (eg. air or water) from the austenitizing temperature (e.g., 17l0 F.) down to thisl transformation range.

In the case of air-hardening alloys, there may be a single step of quenching (e.g. in air) from the austenitizing temperature down to room temperature in the presence of the magnetic iield, at least during the phase transformation temperature range. Y

(E) Stressrelief.-Internal Vstresses induced in the metal by the drastic treatment received inthe three stages 'amants of quenching, are relieved, subsequent to the transformation fix, by a stress relieving operation to which the work may be subjected for a period of time which may range all the way from one hour to 120 hours or more. The terminal temperature as applied to this stress relieving operation may be anywhere from the range of 200 F. to 300 F. for low-alloy steels, and martensitic stainless steels up to a range of 700 F. to 1000 F. for tool steels and chrome steels.

THEORY OF OPERATION OF PROCESS Theory of magnetic quenc1z'izg.-\Vhile I have not obtained conclusive evidence thereof, my experience in the use of my process experimentally over a long period of intensive experimentation and testing, indicates that in the exposure of the highly heated metal to the rapid reduction in temperature attained by a quenching, as in a liquid bath, and the simultaneous subjection of the molecules to high intensity direct magnetic forces while in a highly plastic state derived from the high temperature, the developing martensite platelets are acted upon by the magnetic field as they are transformed from austenite to martensite. This transformation is a progressive one, the percentage of transformation becoming increasingly higher as temperature of the alloy drops toward that of the quench bath. Also, the time of transformation of each martensite platelet from an austenite crystal is extremely short. It appears that the sustained magnetic eld becomes effective on each martensite platelet during its instantaneous transformation from austenite, at the exact instant of transformation to modify its growth in a manner such as to achieve the greatly improved combination of tensile, ductile, malleable and machinability characteristics. In many applications of the process, it appears that the developing crystals are forcibly restrained by the magnetic field from developing the longneedle arrangement which characterizes the conventional austenitic-martensitic type transformation, and are developed into a lattice embodying more uniform spacing between crystals and more uniform geometrical arrangement thereof than is attained in conventional transformations.

At the beginning ofthe magnetic quench, the austenitic crystals are nonmagnetic and apparently are not affected by the magnetic field. Also, they are in a highly plastic state and their resistance to forces affecting the arrangement of developing martensite crystals, is at a minimum. As the temperature drops through the .martensite transformation range, they become magnetic. It appears that the acquisition of magnetic properties sufciently anticipates the actual transformation into martensite crystals, so that the sustained magnetic eld becomes effective upon the crystals during their growth.

As the temperature rapidly descends in the magnetic quench, the crystals, having been constrained to the development of improved lattice condition, are held in that condition by the continuing magnetic field, the increasing resistance to growth and orientation arising from the hardening effect produced by the quench being utilized to resist a return of the crystals toward a large-crystal state characteristic of the ordinary austenitic crystalline structure wherein there is a lack of uniformity both in arrangement andv size of the crystals. Stated somewhat differently, the invention provides for a controlled transformation to the martensiticrstate, the transformation occurring with minimum development of stress levels.

An essential characteristic of the magnetic quench step is the use of a sustained (Le. direct) magnetic field, as contrasted to a fluctuating field such as wouldwresult from the use of alternating current to energize Vthe electromagnetic feld generator (eg. coils). Such a fluctuating field will not produce satisfactory results. l believe that this is because the temporary fixation of the martensitic crystals occurs in a short interval (possibly as short as a few microseconds or within a cycle of current alternation). This is believed to be a possible or partial explanationof the observed effects of a sustained direct current generated magnetic field, as contrasted to an alternating field, which l find to be ineffective to attain the results of the invention.

General theory-While it is my expectation that more information upon the theory of operation of my process will become available as its development progresses, it appears at the present time that some of the characteristics and results obtained may be explained as follows, and for a clearer understanding, reference is now made to FIG. 7 of the drawing:

At the outset, consideration may be given to the well known fact that during a quenching operation, if carried beyond a maximum permissible time interval, there arrives a point where separation between carbon and iron occurs, with a resultant softening effect (separation of ferritic-pearlitic complex). Accordingly, time is a most important factor in a quenching operation, and in order to attain a satisfactory grain structure it is necessary to quench the alloy at the most rapid practical rate for the alloy, until temperature has dropped below the nose of the time-temperature transformation diagram commonly used in representing austenitic-martensitic transformations, so that the martensitic lattice may develop Without diffusion of carbon. lt will of course be understood that for various different alloys, the permissible time interval will vary.

Referring to FIG. 7 specifically, the chart shown therein designates quenching time upon the X-axis, reading from left to right, and designates temperature in degrees Fahrenheit, on the Y-axis. At the upper left hand corner of the chart, the curved broken line t indicates outside temperature of a part being quenched and the curved broken line t designates internal temperature. The point of intersection of the two curved lines, indicated at i, indicates the stage in a quenching operation where these temperatures are equalized.

On the chart, the gap indicated at g designates the point of separation of carbon from iron in a conventional quenching operation, occurring at the maximum time interval indicated at t, the horizontal line Ms designating the temperature at which martensitic transformation begins. In the area F-f-C (ferrite plus carbon) to the right of the separation zone g and above the transformation temperature Ms, the resultant crystalline structure will not have the properties sought in the heat treating process, because of diusion of carbon.

The vertical lines t, l indicate the cooling occurring in the magnetic quench step (eg. cooling from maximum austenitizing temperature to a temperature below the M90 temperature (at which point approximately 90 percent of the austenite has been transformed into martensite).

Y The sinuous broken line w of the chart indicates a period of time during which a part is subjected to the magnetic quench step of my process, and its level indicates the temperature at which this magnetic step is sustained after the initial, quick temperature drop indicated at t, t.

The vertical line d designates the further cooling occurring during the water quench or secondary quench. The sinuous broken line v indicates a further time interval in which the secondary quench takes place, and its level indicates the equilibrium temperature level of this secondary quench step.

The descending line d indicates cooling from room temperature to the range of 400 F. or lower which occurs in the tertiary quench or transformation fix step, and the broken horizontal line z indicates the time interval of such transformation fiX step. The level of this line z indicates the temperature equilibrium attained in this step.

The diagonally ascending broken line r indicates the final stress relieving step wherein the temperature is raised to a level which remains below the tempering level of normal processes, because of the existence of a considerably smaller degree of residual stresses than those which occur in conventionally heat treated metals.

In the refrigerating step, the extremely low temperatures to which the work is subjected result in residual transformation into substantially 100 percent martensite.

In the further and final step of stress relief in which residual stresses are removed, I find that it is possible to attain substantially complete stress relief within a considerably lower temperature range than is utilized in conventional processes, the maximum temperature to which the Work is elevated in my process being several hundred degrees lower than those considered necessary in conventional processes. I attribute this to the fact that the austenitic-martensitic transformation stresses developed in my process are only a small fraction of the extent of such stresses developed in conventional processes.

As the iinal result of my process, I achieve greatly increased tensile strength for any given degree of ductility.

EXAMPLES OF APPLICATION OF PROCESS TO VARIOUS ALLOYS The invention may be more specifically identified by the following specific examples of the process as applied to representative ferromagnetic alloys:

Example I T ype 4130 chrome-moly alloy steel Analysis (percent by weight): carbon 028/033, manganese 0.40/ 0.60; phosphorus,0.04 max.; sulphur 0.04 max.; silicon O20/0.35; nickel none; chromium 0.80/ 1.10; molybdenum 0.15/1.10.

Austenitize to 1625 F.

Magnetic quench in salt bath at 300 F. Magnetic eld intensityvaried from 440 to 900 gauss. Beginning immediately after austenitizing completed. Duration, l min. Terminal temperature-about 300 F.

Secondary quench in Water at room temperature, beginning immediately after completion of magnetic quench. Duration, 1 min. Terminal temperature, room tem perature.

Tertiary quench in refrigerated brine solution at about 100 F., beginning directly after secondary quench. Duration about 3 hours.

Restoration to room temperature, following tertiary quench.

Stress relief-reheat to 212 F. Soak at that temperature for 20 hours and air cool.

Comparative results-Example I as compared to conventional heat treatment of type 4130 alloy steel quenched in oil from 1550 austenitizing temp-tempered by drawing at 800, on a 1 round specimen.

Conventionally Processed Processed by the Invention Yield Strength, p.s.i 190, 000 309,900 average.

288,800 min. 338,400 max. 309,900 average. 288,800 min. 9.0 max. 11 6.6 average.

5.0 min.

{338,400 max.

Tensile Strength, psi 210, 000

Elongation, percent 36.0 max. Reduction of Area, percent 44 2% average.

22.6 min.

Example 2.-Type 4340 nickel-chrome alloy steel Yield, p.s.i. Tensile, Elongation, Reduction p.s.i. percent of Area, percent Conventional Processed 23s, 00o 27o, ooo 1o 44 Processed by the Invention Elongation in 1 inch Test JB847:

405, 200 405, 200 7. 0 19. 6 396, 70 390, 700 7. 0 2l. 1 Test .l B762: i 400, 800 400, 800 4. 0 19. 8 405, 000 405, 000 5. 0 20. 2

Example 3.A.I.S.I. Type C1018 low carbon steel Analysis-percentage of alloying elements: carbon range .l5-.20; manganese .60-.90.

Austenitize to 1750 F.

Magnetic quench in salt bath at 300 F. Magnetic field intensity varied from 400to 900 gauss. Beginning immediately after austenitizing completed. Duration 3 minutes.

Secondary quench-in water at room temperature-begining immediately after magnetic quench completed. Duration about 1 minute.

Tertiary quench-in refrigerated brine at about -'100 F., beginning directly after secondary. Duration, about 3 hours.

Restoration to room temperature, following tertiary quench. Y

Stress relief-reheat to about 212 F. and hold at that temperature for about 30 hours. Then air cool.

(Comparative results cannot be given since'this alloy has not been heretofore regarded as a heat-treatable type of alloy.)

Yield, p.s.i. Tensile, Elongation Reduction p.s.i.` of Area.

Processed by the Invention 211, 500 7. 0 25. 7 Test 113854-- o ge. 3 0 9. l Test H3850-- i 213, 600 6. 0 27. s Test .I B829:

MODIFIED FORMS OF APPARATUS-FIGS. 2-4

suiciently long to house the magnetic quench unit B and the secondary quench unit C shown in FIG. 2, the three tanks being arranged in alignment along the longitudinal axis of the housing 68. Support of the'work table 44a above the heat units A', B and C and successive lowering of the Work into these three units may be provided for by a traveling crane unit 48a including a wheeled carriage 69traveling on track .70 in an upper chamber 71 of the housing 68 and embodying a hoisting cable 72 suspending the work support 44a from a winch 73 actuated by a suitable motor 74. Suitable circulating fans 75 may be utilized to circulate fresh air through the upper compartment 71 so as to maintain a sufiiciently low temperature therein to avoid damage to the crane apparatus. A ceiling 76 of insulating material may be interposed between the heat treatment chamber 67 and the upper compartment 71. The atmosphere in the chamber 67 may be maintained at any selected level between the relatively low temperature utilized in the secondary quench unit C and the relatively high temperature maintained in the salt bath 14a. From the approach conveyor, work may be moved at 17 through the entrance 20 when door 21 thereof is raised, thence onto the work table 44a. The table 44a is then loweredrinto the heated salt bath 14a, allowed to remain there for a period sufiicient to heat the work to the proper temperature, then removed upwardly from the salt bath 14a and transferred horizontally to a position over the quench unit B', thence lowered into that unit for a magnetic quenching operation the same as that described above in connection with the apparatus of FIG. 1, thence raised out of the bath, translated to a position over the secondary quench unit C and lowered into that unit for the secondary quench operation of my process as described above,`thence elevated to a position registering with the exit 27 and deposited upon a pair of tracks 64 projecting into the eXit end of the heat treatment chamber 67. The exit door 28 may then be raised and the work container pulled through the exit 27 into a temperature lock chamber 77 dened within a housing 7S and having a roller conveyor V61a as the hoor thereof and a ame curtain unit 54 operable when the door 28 is raised, to inhibit temperature loss in the heat treatment chamber 67. With a final door79 protecting the exit 80 of the temperature lock housing 78, and a flame curtain unit 81 inhibit-ing loss of temperature in lock chamber 77 when door 79 is raised, the lock unit provides for increased eiciency of temperature retention in the heat treatment chamber 67 by providing for maintaining the outer door 79 closed when the inner door 2S is raised and vice versa, thus avoiding unrestricted out-sweep of atmosphere from the heat treating chamber when the exit door is open. A terminal conveyor 82, positioned in the same plane as conveyor 61a, may be utilized for facilitating the movement of the work away from the outer exit 80.

As shown in FIG.A 3, the magnetic quench unit B' may be of cylindric form, and circulation of liquid within the tank 32a may be provided for by a plurality of radially inwardly directed impellers 57a disposed in a plane adjacent the bottom of the tank. With such a cylindric unit, circular electromagnetic coils 38a and circular cooling coils 41a are utilized.

The secondary quench unit C', as shown in FIG. 2, embodies a quench tank 85 supported within a heat insulator housing 85 and equipped withV a cooling coil 87 extending in successive'convolutions along its inward vertical wall. The unit C', like unit B', may be of cylindric shape. Equipped with suitable automatic thermoresponsive control mechanism, the cooling coil 87 will function to maintain in the water bath with tank 85, the proper temperature (e.g. approximately 200 F.) for the secondary quench operation of my above described process, which is performed in the unit C.

Temperature in the exit lock 77 is maintained at a maximum of, e.g. 200 F.

Parts of the apparatus of FiG. 2 not specifically men- 12 t-ioned are the same as corresponding parts in FIG. 1 and the same numerals are used to designate the same.

As shown in FIG. 4, the magnetic heat-treat unit, instead of having the laterally elongated rectangular form of FIG. 1 or the cylindric form of FIG. 3, may be square in horizontal cross section, the parts otherwise being the same as in FIG. 1 and referred to by the same reference numerals, with the exception of impellers 57a which may be arranged in a manner comparable to that shown in FIG. 3.

MODIFIED FORM-JEIGS. 5 AND 6 FGS. 5 and 6 illustrate an improved form of processing apparatus (specifically, the magnetic quench unit B" thereof) 4which may ybe utilized to carry out the magnetic quench stage of my improved process. Such apparatus includes an outer housing equipped with a temperature regulating coil 91 along its Vertical inner wall, through which iiuid (eg. water) may be circulated .from an inlet 92 to an outlet 93; coaxial intermediate and inward housings 4 and 95 deiining between them an annular cylindric space accommodating the magnetic coil 38b, a chamber 40a being defined within housing 95; a ring gear 96 attached to the upper end of and suspending the housings 94 and 95 and coils 38h; annular roller support means 97 interposed lbetwecn the ring gear 96 and ceiling member 9S of outer housing 90 and providing anti friction rolling support for the suspended parts; a delivery conduit 99 extending downwardly through the ring gear 96 and into the heat treatment chamber 40a defined within the inward housing 95, for'dropping into the heat treatment bath within chamber 40a, the heated metal parts of gates 1G41, hinged at 101 to the bottom of inward housing 96 and normally disposed in closed positions in a common horizontal plane to provide a closed bottom for the chamber 40a upon -which the work may be supported during the magnetic quench step suitable actuator means, including swinging lift arms 102 utilized for periodically dropping the gates 100 to the positions indicatedy in broken lines in F-G 5; suitable vertically extending agitator vanes 132 attached to the inner wall of housing and projecting inwardly and vanes 104 attached to the outward wall of outer housing 94 and projecting outwardly, for agitating the liquid bath permeating the entire area Within the outer housing 90 and circulating through apertures `ltfS in the walls 94 and 95; supplementary bath-circulating impellers 106 in a bottom chamber 107 below the bottom of the central rotating unit 89 and driven by respective motors; and an elevating conveyor which can be of the endless belt of endless chain type, including a horizontal unit 108 disposed in the bottom chamber 107, in a position to receive from the rotating unit S the work articles deposited thereon by dropping the gates 1%, and an elevating unit 109 for elevating the work lfrom the lower horizontal conveyor yunit 163 to a level above the housing 90 through a suitable exit opening 110 in the ceiling 98 thereof, and a carry-away section 111 `for transporting the articles of work to the secondary quench unit C. The ring gear 96 is'driven by a suitable drive pinion 112 through a drive shaft 113 driven from a suitable motor and reduction gear (not sho-wn).

Iclaim:

i1. An improved process of etecting an austenite to martensite transformation in an alloy of the austeniticmartensitic type, including ythe following steps: heating the article to an austenitizing temperature; and then quenching the article rapidly in a quenching medium maintained at approximately the temperature of the lower limit of the transformation range for the alloy while subjecting it to a sustained-intensity magnetic iield of at least gauss for a period of time not substantially less than l0 minutes.

2. An improvedv processV of effecting an austenite to martensite transformation in a ferromagnetic alloy of the austenitic-martensitic type, including the following steps: austenitizing the alloy; then rapidly quenching the alloy through its critical transformation range while subjecting it to a sustained intensity magnetic iield of at -least 100 gauss throughout said range; and arresting the quenching operation at a temperature approximately at the lower limit of said range.

3. An improved process of effecting an austenite to martensite transforma-tion in a ferromagnetic alloy of the austenitic-rnatensitic type, including the following steps: heating the article to a temperature higher than the austenitizing temperature for that metal but below its melting point; and then quenching the article rapidly in a liquid quenching medium maintained at a temperature just below the lower limit of the austenitic-martensitic transformation range of the alloy while subjecting it to a sustained-intensity magnetic field of intensity in the range 750 to i500 gauss for a period of not substantially less than minutes.

4. An improved process of effecting an austenite to martensite transformation in a ferromagnetic alloy of the austenitic-martensitic type, including the following steps: austenitizing lthe allor; then rquenching the alloy rapidly through its critical transformation range above 200 in a quenching bath having a temperature maintained at a temperature near the lower limit of said transformation range, while subjecting it to a sustained high intensity magnetic field of intensity in the range 750 to 1500 gauss throughout said range; and then subjecting the alloy to a more rapid quench down to room temperature.

5. An improved process of effecting an austenite to martensite transformation in a ferromagnetic alloy of the austenitic-martensitic type, including the fol-lowing steps: austenitizing the alloy at a temperature higher than the recognized austenitizing temperature for that alloy; then `quenching the austenitized alloy rapidly through its austenite-to-martensite transformation range in a quench medium maintained at a tempera-ture just below the lower llimit of said transformation range while constantly subjecting it to a direct-current induced sustained high intensity magnetic field of at least gauss intensity throughout its passage through said range; then immediately subjecting the alloy to a more rapid quench .cooling it to room temperature, whereby to develop in said alloy, tensile strength in the range of 1A to 1/: higher than the tensile strength of, and with as good ductility and malleability of the alloy as developed in optimum existing heat-treating process.

y6. The process defined in claim 5, including the further subsequent step of subjecting the alloy to refrigeration at a sub-zero F. temperature rfor a -period of time sufficient to stabilize its transformed crystalline structure.

7. The process defined in claim 6, including the further "subsequent step of reheating the stabilized alloy to a temperature above the normal stress-relief temperature for .the alloy, and maintaining at that temperature for a .period of time such as to relieve residual stresses while preserving said tensile, ductility and malleability characteristics.

References Cited in the le of this patent UNITED STATES PATENTS 475,498 IFraley May 24, 1892 1,171,832 Bishop Feb. 15, 1916 1,895,998 Knerr Jan. 31, 1933 1,978,221 Otte Oct. 23, 1934 2,363,741 Montgomery NOV. 28, 1944 2,686,865 Kelly Aug. 17, 1954 2,867,226 Williams et al. Jan. 6, 1959 2,874,952 Dammert et al. Feb. 24, 1959 2,914,311 Yarne NOV. 24, 1959 2,965,525 Burbank et al. Dec. 20, 1960 3,024,142 Parkin Mar. 6, 1962 FOREIGN PATENTS 572,409 Great Britain Oct. 8, 1945 OTHER REFERENCES The Iron Age, vol. 153, Apr. 13, 1944, pages 52-55. The Making, Shaping and Treating of Steel, United States, seventh edition, 1957, page 827. 

1. AN IMPROVED PROCESS OF EFFECTING AN AUSTENITE TO MARTENSITE TRANSFORMATION IN AN ALLOY OF THER AUSTENITICMARTENSITIC TYPE, INCLUDING THE FOLLOWING STEPS: HEATING THE ARTICLE TO AN AUSTENITIZING TEMPERATURE; AND THEN QUENCHING THE ARTICLE RAPIDLY IN A QUENCHING MEDIUM MAINTAINED AT APPROXIMATELY THE TEMPERATURE OF THE LOWER LIMIT OF THE TRANSFORMATION RANGE FOR THE ALLOY WHILE 